It could very well be copper I wonder how your test it
A simple rub with a soft cotton cloth in an inconspicuous area would reveal the yellow color of brass underneath. If the color remains reddish copper it's not brass.
I seriously doubt it's brass, being in the water, the zinc would have migrated to the surface and you would see gray/green/white crumbly residue on the exterior instead of the red color of copper.
During the period your design was manufactured the better ones were made with a copper body and a brass measuring spout. It was believed that combination was sparkless (an important consideration!) and the copper was less prone to corrosion from the powder than brass.
As far as conservation the first step would be to get the top unscrewed and give the inside a thorough cleaning with clean
cold water. The Potassium nitrate in the gunpowder residue will have displaced most of the more soluble copper sulfate in that wet environment. Hot water will put any copper sulfate still in the flask into solution and you really don't want to lose that while there are still nitrates in the flask.
The washing should be a slow gentle process. Keep the flask under clean cold water without any agitation or rubbing. Change the water every day or so. Try not to disturb the flask while you are changing the water. This is no time to use a hose or faucet, just gently and slowly fill the soak container from a pitcher. A bottom drain will be helpful. This process may take a week or a month or a year. Patience is the key to success. Placing the flask on a removable rack will make the whole process much easier and you won't have to handle the flask to remove it from the water bath.
If you start to see a green tinge on the copper your water is not clean enough, you will have to use distilled water if that's the case.
Once you think you have gotten most, or all, of the residue from the inside shine a bright light into the flask in a very dark room. Look for pinholes and areas where you can see some light through the metal. If the flask shows no light and the gunpowder residue is gone you can move on to final conservation or restoration. You will have to decide which you want to do because the copper will continue to deteriorate if you just leave it in it's current state.
